The present invention relates to a process for pre-reforming of a process gas e.g. natural gas, peak shaving gas, LPG etc. containing oxygen. Pre-reforming is a process being carried out prior to catalytic steam reforming in the production of hydrogen and/or carbon monoxide rich gas. In the pre-reforming step, content of higher hydrocarbons is converted to methane and the methane reforming reaction (1) and the water-gas shift reaction are equilibrated close to the operating temperature.
Oxygen in the feed gas to the above process results in oxygenated dehydration of the hydrocarbons and produces olefins. Pre-reforming catalysts are critical towards olefin concentration in the gas and will form carbon when the concentration exceeds certain critical values.
Partial oxidation of hydrocarbon feed with H2O or CO2 in presence of oxygen over a transition metal catalyst and a noble metal catalyst is mentioned in WO 99/35082.
The general object of this invention is thus to improve the known processes for catalytic steam reforming of a hydrocarbon feedstock containing oxygen including a step of catalytically pre-reforming the feedstock without substantially formation of detrimental carbon on the pre-reforming catalyst.
In particular, the invention makes use of a noble metal catalyst being active in catalytic oxidation using the oxygen available in the inlet gas to oxidise hydrocarbons to carbon monoxide e.g. (3) and carbon dioxide e.g. (4) and having activity as a pre-reformer catalyst converting higher hydrocarbons to methane and equilibrating the methane ane reforming reaction (1) and water gas shift reaction (2). 
Exothermic catalytic oxidation reactions 3 and 4 are the prevailing reactions at inlet of a pre-reformer and release heat to the subsequent reactions, resulting in temperature increase of the process gas dependent on the oxygen content in the feed gas. When the content of oxygen in the process gas is consumed by the above reactions 3 and 4, the endothermic methane reforming reaction will be the main reaction leading to a decrease in process temperature. Because of heat formation through catalytic combustion reactions 3 and 4, the outlet gas will have a higher temperature and a higher methane reforming equilibrium temperature than in steam reforming process without oxygen in the inlet feed. The subsequent steam reforming step requires therefore less duty to obtain the same amount of hydrogen and/or carbon monoxide in the effluent gas from the steam reforming step.
In accordance with the above observations, the invention provides a process for the catalytic steam reforming of a hydrocarbon feed stock with a content of higher hydrocarbons and oxygen comprising the steps of catalytically pre-reforming the feed stock and steam reforming the pre-reformed feed stock. The pre-reforming step is carried out in presence of a fixed bed catalyst comprising at least a portion of a noble metal catalyst being active in oxidation of hydrocarbons to carbon oxides and conversion of higher hydrocarbons to methane. The noble metal catalyst is supported on MgO and/or MgAl2O4 spinel.
Suitable metals for use in the oxidation catalyst include at least one noble metal selected from Group VIII of the Periodic Table supported on a catalyst carrier of alumina, magnesia, titania, silica, zirconia, beryllia, thoria, lanthania, calcium oxide or compounds of mixtures thereof.
Catalysts containing noble metal for use in the process according to the invention are conveniently prepared by any of the methods conventionally used in the art, including impregnation of a carrier material with a noble metal containing aqueous solution and calcinating the impregnated material in air.
Carrier materials are usually selected from the group of alumina, magnesia, titania, silica, zirconia, beryllia, thoria, lanthania, calcium oxide and compounds of mixtures thereof. Preferred carrier materials are magnesium, alumina spinel and magnesium oxide.
The noble metal catalyst is typically arranged as fixed bed in a pre-reformer reactor.
Optionally, the noble catalyst may be employed as a top-layer in the pre-reformer reactor on a conventional pre-reforming catalyst.